Combustion chemical vapor deposition ("CCVD"), a recently invented CVD technique, allows for open atmosphere deposition of thin films. The CCVD process offers several advantages over other thin-film technologies, including traditional CVD. The key advantage of CCVD is its ability to deposit films in the open atmosphere without any costly furnace, vacuum, or reaction chamber. As a result, the initial system capitalization requirement can be reduced up to 90% compared to a vacuum based system. Instead of a specialized environment, which is required by other technologies, a combustion flame provides the necessary environment for the deposition of elemental constituents from solution, vapor, or gas sources. The precursors are generally dissolved in a solvent that also acts as the combustible fuel. Depositions can be performed at atmospheric pressure and temperature within an exhaust hood, outdoors, or within a chamber for control of the surrounding gasses or pressure.
Because CCVD generally uses solutions, a significant advantage of this technology is that it allows rapid and simple changes in dopants and stoichiometries which eases deposition of complex films. The CCVD technique generally uses inexpensive, soluble precursors. In addition, precursor vapor pressures do not play a role in CCVD because the dissolution process provides the energy for the creation of the necessary ionic constituents. By adjusting solution concentrations and constituents, a wide range of stoichiometries can be deposited quickly and easily. Additionally, the CCVD process allows both chemical composition and physical structure of the deposited film to be tailored to the requirements of the specific application.
Unlike conventional CVD, the CCVD process is not confined to an expensive, inflexible, low-pressure reaction chamber. Therefore, the deposition flame, or bank of flames, can be moved across the substrate to easily coat large and/or complex surface areas. Because the CCVD process is not limited to specialized environments, the user can continuously feed materials into the coating area without disruption, thereby permitting batch processing. Moreover, the user can limit deposition to specific areas of a substrate by simply controlling the dwell time of the flame(s) on those areas. Finally, the CCVD technology generally uses halogen-free chemical precursors having significantly reduced negative environmental impact.
Numerous materials have been deposited via CCVD technology with the combustion of a premixed precursor solution as the sole heat source. This inexpensive and flexible film deposition technique permits broad use of thin film technology. The CCVD process has much of the same flexibility as thermal spraying, yet creates quality, conformal films like those associated with conventional CVD. With CCVD processing, a desired phase can be deposited in a few days and at relatively low cost.
A preferred embodiment of the CCVD process is described in detail in U.S. Application Ser. No. 08/691,853 filed Aug. 2, 1996, the teachings of which are incorporated herein by reference. In accordance with that application, a CCVD produces vapor formed films, powders and nanophase coatings from near-supercritical liquids and supercritical fluids. Preferably, a liquid or liquid-like solution fluid containing chemical precursor(s) is formed. The solution fluid is regulated to near or above the critical pressure and is then heated to near the supercritical temperature just prior to being released through a restriction or nozzle which results in a gas entrained very finely atomized or vaporized solution fluid. The solution fluid vapor is combusted to form a flame or is entered into a flame or electric torch plasma, and the precursor(s) react to the desired phase in the flame or plasma or on the substrate surface. Due to the high temperature of the plasma much of the precursor will react prior to the substrate surface. A substrate is positioned near or in the flame or electric plasma, and a coating is deposited. Alternatively, the material formed can be collected as a nanophase powder.
Very fine atomization, nebulization, vaporization or gasification is achieved using solution fluids near or above the critical pressure and near the critical temperature. The dissolved chemical precursor(s) need not have high vapor pressure, but high vapor pressure precursors can work well or better than lower vapor pressure precursors. By heating the solution fluid just prior to or at the end of the nozzle or restriction tube (atomizing device), the available time for precursor chemical reaction or dissolution prior to atomization is minimized. This method can be used to deposit coatings from various metalorganics and inorganic precursors. The fluid solution solvent can be selected from any liquid or supercritical fluid in which the precursor(s) can form a solution. The liquid or fluid solvent by itself can consist of a mixture of different compounds.
A reduction in the supercritical temperature of the reagent containing fluid produces superior coatings. Many of these fluids are not stable as liquids at STP, and must be combined in a pressure cylinder or at a low temperature. To ease the formation of a liquid or fluid solution which can only exist at pressures greater than ambient, the chemical precursor(s) are optionally first dissolved in primary solvent that is stable at ambient pressure. This solution is placed in a pressure capable container, and then the secondary (or main) liquid or fluid (into which the primary solution is miscible) is added. The main liquid or fluid has a lower supercritical temperature, and results in a lowering of the maximum temperature needed for the desired degree of nebulization. By forming a high concentration primary solution, much of the resultant lower concentration solution is composed of secondary and possible additional solution compounds. Generally, the higher the ratio of a given compound in a given solution, the more the solution properties behave like that compound. These additional liquids and fluids are chosen to aid in the very fine atomization, vaporization or gasification of the chemical precursor(s) containing solution. Choosing a final solution mixture with low supercritical temperature additionally minimizes the occurrence of chemical precursors reacting inside the atomization apparatus, as well as lowering or eliminating the need to heat the solution at the release area. In some instances the solution may be cooled prior to the release area so that solubility and fluid stability are maintained. One skilled in the art of supercritical fluid solutions could determine various possible solution mixtures without undue experimentation. Optionally, a pressure vessel with a glass window, or with optical fibers and a monitor, allows visual determination of miscibility and solute-solvent compatibility. Conversely, if in-line filters become clogged or precipitant is found remaining in the main container, an incompatibility under those conditions may have occurred.
Another advantage is that release of fluids near or above their supercritical point results in a rapid expansion forming a high speed gas-vapor stream. High velocity gas streams effectively reduce the gas diffusion boundary layer in front of the deposition surface which, in turn, improves film quality and deposition efficiency. When the stream velocities are above the flame velocity, a pilot light or other ignition means must be used to form a steady state flame. In some instances two or more pilots may be needed to ensure complete combustion. With the plasma torch, no pilot lights are needed, and high velocities can be easily achieved by following operational conditions known by one of ordinary skill in the art.
The solute-containing fluid need not be the fuel for the combustion. Noncombustible fluids like water N.sub.2 O or CO.sub.2, or difficult to combust fluids like ammonia, can be used to dissolve the precursors or can serve as the secondary solution compound. These are then expanded into a flame or plasma torch which provides the environment for the precursors to react. The depositions can be performed above, below or at ambient pressure. Plasma torches work well at reduced pressures. Flames can be stable down to 10 torr, and operate well at high pressures. Cool flames of even less than 500.degree. C. can be formed at lower pressures. While both can operate in the open atmosphere, it can be advantageous to practice the methods of the invention in a reaction chamber under a controlled atmosphere to keep airborne impurities from being entrained into the resulting coating. Many electrical and optical coating applications require that no such impurities be present in the coating. These applications normally require thin films, but thicker films for thermal barrier, corrosion and wear applications can also be deposited.
Further bulk material can be grown, including single crystals, by extending the deposition time even further. The faster epitaxial deposition rates provided by higher deposition temperatures, due to higher diffusion rates, can be necessary for the deposition of single crystal thick films or bulk material.
CCVD is a flame process which utilizes oxygen. While it may be possible using CCVD to deposit oxygen-reactive materials with CCVD by depositing in the reducing portions of the flame, a better technique for depositing oxygen reactive materials, such as nickel, is a related process described in U.S. patent application Ser. No. 09/067,975, filed the same date as the instant application, the teachings of which are incorporated herein by reference.
The invention described in referenced U.S. patent application Ser. No. 09/067,975 provides an apparatus and method for chemical vapor deposition wherein the atmosphere in a coating deposition zone is established by carefully controlling and shielding the materials fed to form the coating and by causing the gases removed from the deposition zone to pass through a barrier zone wherein they flow away from said deposition zone at an average velocity greater than 50 feet per minute, and preferably greater than 100 feet per minute. The rapid gas flow through the barrier zone essentially precludes the migration of gases from the ambient atmosphere to the deposition zone where they could react with the coating or the materials from which the coating is derived. Careful control of the materials used to form the coating can be provided by feeding the coating precursors in a fixed proportion in a liquid media. The liquid media is atomized as it is fed to a reaction zone wherein the liquid media is vaporized and the coating precursors react to form reacted coating precursors. Alternatively, the coating precursor(s) can be fed as a gas, either as itself or as a mixture in a carrier gas. The reacted coating precursors are often composed of partially, fully and fractionally reacted components, which can flow as a plasma to the deposition zone. The reacted coating precursors contact and deposit the coating on the surface of the substrate in the deposition zone. A curtain of flowing inert gases may be provided around the reaction zone to shield the reactive coating materials/plasma in that zone from contamination with the materials used in the surrounding apparatus or with components of the ambient atmosphere.
The vaporization of the liquid media and reaction of the coating precursors in the reaction zone requires an input of energy. The required energy can be provided from various sources, such as electrical resistance heating, induction heating, microwave heating, RF heating, hot surface heating and/or mixing with hot inert gas.
Herein, non-combustion process will be referred to as "Controlled Atmosphere Combustion Chemical Vapor Deposition" (CACCVD). This technique provides a relatively controlled rate of energy input, enabling high rates of coating deposition. In some preferred cases, the liquid media and/or a secondary gas used to atomize the liquid media can be a combustible fuel used in the CACCVD. Particularly important is the capability of CACCVD to form high quality adherent deposits at or about atmospheric pressure, thereby avoiding the need to be conducted in elaborate vacuum or similar isolation housings. For these reasons, in many cases, CACCVD thin film coatings can be applied in situ, or "in the field", where the substrate is located.
Combustion chemical vapor deposition (CCVD) is not suitable for those coating applications which require an oxygen free environment. For such applications, CACCVD, which employs non-combustion energy sources such as hot gases, heated tubes, radiant energy, microwave and energized photons as with infrared or laser sources are suitable. In these applications it is important that all of the liquids and gases used be oxygen-free. The coating precursors can be fed in solution or suspension in liquids such as ammonia or propane which are suitable for the deposit of nitrides or carbides, respectively.
CACCVD processes and apparatus provide control over the deposition zone atmosphere, thereby enabling the production of sensitive coatings on temperature sensitive or vacuum sensitive substrates, which substrates can be larger than could otherwise be processed by conventional vacuum chamber deposition techniques.
A further advantage of CACCVD is its ability to coat substrates without needing additional energy supplied to the substrate. Accordingly, this system allows substrates to be coated which previously could not withstand the temperatures to which substrates were subjected by most previous systems. For instance, nickel coatings can be provided on polyimide sheet substrates without causing deformation of the substrate. Previously atmospheric pressure deposition techniques were unable to provide chemical vapor deposition of metallic nickel because of its strong affinity to oxygen, while vacuum processing of polyimide sheet substrates was problematical due to its outgassing of water and tendency toward dimensional instability when subjected to heat and vacuum.